Add a new option to newfs(8), -r, to specify reserved space at the
end of the device. It can be useful, e.g., when the device is to
become a member of a gmirror array later w/o losing the file system
on it.
Document the new option in the manpage.
While I'm here, improve error handling for -s option, which is
syntactically similar to -r; and document the fact that -s0 selects
the default fs size explicitly, which can be useful, e.g., in a
menu-based wrapper around newfs(8) requiring some value be entered
for the fs size.
Also fix a small typo in the help line for -s (missing space).
Idea and initial implementation by: marck
Discussed on: -fs
Critical review by: bde
Tested with: cmp(1)
13b1c70fac